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Abstract:

There is provided a method for manufacturing a honeycomb filter, the
method including: a step of depositing plugging material particles which
burn away due to a thermal treatment on the inflow cell side surface
layer portion of a honeycomb-shaped substrate having porous partition
walls separating and forming plural cells, and plugging portions, a step
of depositing membrane-forming particles on the surface layer portion
where the plugging material particles deposit, and a step of subjecting
the honeycomb-shaped substrate having the plugging material particles and
membrane-forming particles depositing on the partition walls thereof to a
thermal treatment. The number average particle diameter of the first
particles is at most an average pore size of the pores formed in the
partition walls.

Claims:

1. A method for manufacturing a honeycomb filter, the method comprising:
a first step of plugging openings of pores of a honeycomb-shaped
substrate having porous partition walls separating and forming a
plurality of cells functioning as exhaust gas fluid passages and having a
large number of pores formed therein, and plugging portions disposed in
end portions on one side of exhaust gas inflow cells among the cells and
in end portions on the other side of exhaust gas outflow cells adjacent
to the inflow cells by allowing first particles which burn away due to a
thermal treatment to deposit in at least the pores open on the inflow
cell side, a second step of allowing second particles for manufacturing a
membrane to deposit on the partition walls where the opening of the pores
are plugged, and a third step of subjecting the honeycomb-shaped
substrate having the first and second particles depositing on the
partition walls thereof to a thermal treatment; wherein a number average
particle diameter of the first particles is at most an average pore size
of the pores formed in the partition walls.

2. The method for manufacturing a honeycomb filter according to claim 1,
wherein the number average particle diameter of the first particles is at
most 0.50 times the average pore size of the pores formed in the
partition walls.

3. The method for manufacturing a honeycomb filter according to claim 1,
wherein the number average particle diameter of the first particles is
0.0007 to 0.50 times the average pore size of the pores formed in the
partition walls.

Description:

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The present invention relates to a method for manufacturing a
honeycomb filter for trapping particulate matter contained in exhaust
gas.

[0002] In consideration of influences on the environment, the need for
removing particulate matter contained in exhaust gas discharged from
internal combustion engines such as a vehicle engine, a construction
machine engine, and an industrial machine stationary engine, other
burning appliances, and the like is increasing. In particular,
regulations relating to removal of particulate matter (hereinbelow
sometimes referred to as "PM") discharged from a diesel engine tend to be
strengthened on a global basis. From such circumstances, attention is
paid to a DPF (diesel particulate filter) for trapping and removing the
PM.

[0003] One embodiment of a DPF is a honeycomb filter 1 provided with
porous partition walls 12 separating and forming a plurality of cells 11
functioning as exhaust gas flow passages and having a large number of
pores formed therein and plugging portions 13a arranged in the outflow
end portions 15b of the exhaust gas inflow cells 11a and in the inflow
end portions 15a of the exhaust gas outflow cells 11b adjacent to the
inflow cells 11a among the cells 11 as shown in FIGS. 1 and 2. In such a
honeycomb filter 1, exhaust gas flowing in from the inflow end portions
15a where the inflow cells 11a are open passes through the partition
walls 12, flows into the outflow cells 11b, and is discharged from the
outflow end portions 15b where the outflow cells 11b are open, thereby
trapping and removing PM in the exhaust gas due to the partition walls
12. In such a filter having a structure where exhaust gas passes through
the porous partition walls 12 (wall flow type filter), since a filtration
area can be secured widely, a filtration flow rate (partition wall
transmission flow rate) can be reduced to have small pressure loss and
relatively good PM-trapping efficiency.

[0004] Since a DPF has such a structure as described above, when trapping
of PM is started in a clean state without deposition of PM or the like,
PM deposits in the pores inside the partition walls, which may rapidly
increase pressure loss. Such rapid increase in pressure loss may become a
major factor of deterioration of engine performance. In order to solve
the problem and in order to enhance the PM-trapping efficiency, there has
been proposed a honeycomb filter having a trapping layer fixed on the
surfaces of the partition wall by sending ceramic particles into the
cells with gas (see, e.g., JP-A-2006-685).

[0005] However, since a pressure distribution is generated by an air
current and since ceramic particles themselves have an inertia force when
gas containing ceramic particles is sent into the cells, as shown in FIG.
3, ceramic particles easily deposit in the pores of the partition walls
in the outflow end portions 15b of the inflow cells 11a and further on
the surfaces of the partition walls 12. As the results, the membrane
thickness of the resultant trapping layers 30 becomes nonuniform, i.e.,
small in the inflow end portions 15a and large in the outflow end
portions 15b to have a problem of difficulty in forming a trapping layer
having a small and uniform thickness.

[0006] In order to solve the problem, for example, JP-A-10-263340
discloses formation of a uniform trapping layer by inserting a divider
and a brush into a cell, sending ceramic particles with gas into a
circular cylindrical portion partitioned by two dividers, and sweeping
the surface of the partition walls with the brush. However, in the case
of a large number of small cells as in a DPF or the like, there is a
problem of impractical formation of a uniform trapping layer according to
such a method.

SUMMARY OF THE INVENTION

[0007] The present invention has been made in view of such prior art
problems and aims to provide a method for manufacturing a honeycomb
filter which is provided with a trapping layer having a small and uniform
thickness and which has excellent pressure loss properties.

[0008] As a result of the present inventors' earnest investigations in
order to achieve the aforementioned aim, they found out that the
aforementioned aim can be achieved by depositing the plugging material
particles which burn away due to a thermal treatment in the pores in the
surface layer portions of the partition walls to make the partition wall
surfaces almost flat, then thinly and uniformly depositing
membrane-forming particles thereon, and then burning away the plugging
material particles due to the thermal treatment. The finding has lead to
the completion of the present invention.

[0009] That is, according to the present invention, there is provided the
following method for manufacturing a honeycomb filter.

[0010] [1] A method for manufacturing a honeycomb filter, the method
comprising: a first step of plugging openings of pores of a
honeycomb-shaped substrate having porous partition walls separating and
forming a plurality of cells functioning as exhaust gas fluid passages
and having a large number of pores formed therein, and plugging portions
disposed in end portions on one side of exhaust gas inflow cells among
the cells and in end portions on the other side of exhaust gas outflow
cells adjacent to the inflow cells by allowing first particles which burn
away due to a thermal treatment to deposit in at least the pores open on
the inflow cell side, a second step of allowing second particles for
manufacturing a membrane to deposit on the partition walls where the
opening of the pores are plugged, and a third step of subjecting the
honeycomb-shaped substrate having the first and second particles
depositing on the partition walls thereof to a thermal treatment; wherein
a number average particle diameter of the first particles is at most an
average pore size of the pores formed in the partition walls.

[0011] [2] The method for manufacturing a honeycomb filter according to
[1], wherein the number average particle diameter of the first particles
is at most 0.50 times the average pore size of the pores formed in the
partition walls.

[0012] [3] The method for manufacturing a honeycomb filter according to
[1], wherein the number average particle diameter of the first particles
is 0.0007 to 0.50 times the average pore size of the pores formed in the
partition walls.

[0013] According to a method for manufacturing a honeycomb filter of the
present invention, there can be provided a method for manufacturing a
honeycomb filter which is provided with a trapping layer having a small
and uniform thickness and which has excellent pressure loss properties.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a front (inflow end face) view schematically showing an
embodiment of a conventional DPF.

[0016]FIG. 3 is a cross-sectional view schematically showing an inflow
cell of a honeycomb filter obtained by a conventional method for
manufacturing a honeycomb filter.

[0017]FIG. 4A is a cross-sectional view schematically showing an inflow
cell after being subjected to the step 1 in an embodiment of a method for
manufacturing a honeycomb filter of the present invention.

[0019]FIG. 5A is a cross-sectional view schematically showing an inflow
cell after being subjected to the step 2 in an embodiment of a method for
manufacturing a honeycomb filter of the present invention.

[0021] FIG. 6A is a cross-sectional view schematically showing an inflow
cell after being subjected to the step 3 in an embodiment of a method for
manufacturing a honeycomb filter of the present invention.

[0024] Hereinbelow, embodiments of the present invention will be
described. However, the present invention is by no means limited to the
following embodiments, and it should be understood that the present
invention includes embodiments where changes, improvements and the like
are suitably made on the following embodiments on the basis of the
ordinary knowledge of a person of ordinary skill in the art within the
range of not deviating from the gist of the present invention.

[0025] 1: Method for Manufacturing a Honeycomb Filter:

[0026] An embodiment of a method for manufacturing a honeycomb filter of
the present invention is a manufacturing method where a honeycomb-shaped
substrate is subjected to the steps 1 to 3. Hereinbelow, the details will
be described.

[0027] 1-1. Manufacture of Honeycomb-Shaped Substrate:

[0028] The honeycomb-shaped substrate of the present embodiment can be
manufactured by the following method. In the first place, framework
particles of a ceramic or the like shown below, water, a pore former, and
the like are mixed together, and the mixture is kneaded to obtain kneaded
clay. Next, the kneaded clay is formed into a desired honeycomb shape due
to extrusion forming or the like and then dried to obtain a honeycomb
formed article. Next, plugging portions are formed in one of the end
portions of the predetermined cells of the honeycomb formed article.
Finally, firing is performed to obtain a honeycomb-shaped substrate
having porous partition walls separating and forming a plurality of cells
which function as exhaust gas flow passages and having a large number of
pores, and plugging portions disposed in end portions on one side of
exhaust gas inflow cells among the cells and in end portions on the other
side of exhaust gas outflow cells adjacent to the inflow cells.

[0029] In addition, there is no particular limitation on the material for
the honeycomb-shaped substrate of the present embodiment, and
conventionally known materials can be used. Among the materials, ceramics
such as cordierite, silicon carbide (SiC), Si--SiC, alumina, mullite,
aluminum titanate, and silicon nitride are preferable, and Si--SiC,
cordierite, aluminum titanate, and the like are particularly preferable.

[0030] The porosity of the partition walls of the honeycomb-shaped
substrate is preferably 35 to 75%. When the porosity of the partition
walls is below 35%, since permeation resistance of the partition walls
upon filtering exhaust gas remarkably rises, pressure loss in a state
without PM deposition increases to a large extent. On the other hand,
when the porosity is above 75%, strength of the honeycomb-shaped
substrate falls, and a crack may be generated upon canning.

[0031] The average pore size of the pores formed in the partition walls of
a honeycomb-shaped substrate is preferably 5 to 40 μm. When the
average pore size of the pores is below 5 μm, since permeation
resistance of the partition walls upon filtering exhaust gas remarkably
rises, pressure loss in a state without PM deposition increases to a
large extent. On the other hand, when the average pore size of the pores
is above 40 μm, the amount of PM passing through the partition walls
increases to a large extent, and the filtering performance may
deteriorate.

[0032] 1-1-1. Preparation of Kneaded Clay:

[0033] The kneaded clay can be prepared by mixing framework particles of a
ceramic or the like described above; water as a dispersion medium, a pore
former such as graphite, starch, or synthetic resin, and the like, and
kneading the mixture. In addition, with the kneaded clay, an organic
binder, a dispersant, and the like may further be mixed arbitrarily.

[0034] As the framework particles, there may be employed, for example, a
mixture of a SiC powder and a metal Si powder mixed at a mass ratio of,
for example, 80:20 when a Si--SiC honeycomb-shaped substrate is
manufactured; and, for example, a cordierite-forming raw material or the
like containing a plurality of inorganic raw materials selected from the
group consisting of talc, kaolin, calcined kaolin, alumina, aluminum
hydroxide, and silica so as to have a chemical composition of, for
example, silica of 42 to 56 mass %, alumina of 30 to 45 mass %, and
magnesia of 12 to 16 mass % when a cordierite honeycomb-shaped substrate
is manufactured.

[0035] There is no particular limitation on the pore former as long as it
flies apart or burns away by the firing (calcination) step, and
conventionally known pore formers may be used. Specific examples of the
pore formers include inorganic substances such as coke, polymer compound
such as resin balloons, and organic substance such as starch.
Incidentally, these pore formers may be used alone or as a combination of
two or more kinds.

[0036] Specific examples of the organic binder include hydroxypropylmethyl
cellulose, methyl cellulose, hydroxyethyl cellulose, carboxylmethyl
cellulose, and polyvinyl alcohol. These organic binders may be used alone
or as a combination of two or more kinds.

[0037] Specific examples of the dispersant include ethylene glycol,
dextrin, fatty acid soap, and polyalcohol. These dispersants may be used
alone or as a combination of two or more.

[0038] There is no particular limitation on a method for kneading a
mixture of the aforementioned materials, and a conventionally known
method may be employed. For example, a method using a kneader, a vacuum
kneader, or the like may be employed.

[0039] 1-1-2. Formation:

[0040] There is no particular limitation on the method for forming a
honeycomb formed article, and a conventionally known method may be
employed. For example, methods such as extrusion forming, injection
forming, press forming, and the like may be employed. Of these,
particularly preferable is a method of subjecting kneaded clay prepared
as described above to extrusion forming using a die having a desired cell
shape, partition wall thickness, and cell density.

[0041] There is no particular limitation on the entire shape of the
honeycomb formed article, and a conventionally known shape may be
employed. For example, shapes such as a circular columnar (circular
cylindrical) shape, an elliptic columnar shape, a quadrangular prism
shape, and a triangular prism shape may be employed.

[0042] There is no particular limitation on the cell structure of a
honeycomb formed article. The cell density of the honeycomb-shaped
substrate is preferably 0.9 to 233 cells/cm2, and the partition wall
thickness of honeycomb-shaped substrate is preferably 100 to 600 μm.
When the partition wall thickness is below 100 μm, a crack may be
caused upon regeneration of the DPF. On the other hand, when the
partition wall thickness is above 600 μm, the cell has a small
equivalent hydraulic diameter, and the pressure loss may increase.

[0043] There is no particular limitation on the cell shape of the
honeycomb formed article, and a conventionally known shape may be
employed. For example, cell shapes having cross sectional shapes of a
quadrangle, a hexagon, an octagon, and a triangle may be employed. In
addition, in the honeycomb formed article, plural kinds of cell shapes
having different cross sections or sizes may be formed.

[0044] 1-1-3. Plugging:

[0045] The plugging portions of the honeycomb-shaped substrate of the
present embodiment can be formed by, for example, immersing a honeycomb
formed article having a mask on the cells where no plugging portion is
formed in stored plugging material slurry for the aimed thickness of the
plugging portions to fill the plugging material slurry into the plugging
cells. Incidentally, after the plugging material slurry is filled into
the cells to be plugged, generally, the honeycomb formed article is
lifted out and dried, and the mask is removed. In addition, in the same
manner, plugging portions can be formed at the end portions on the other
side of the masked cells.

[0046] As the plugging material, the same material as the material for the
honeycomb formed article is generally used. By using the same material as
the material for the honeycomb formed article, the plugging material and
the honeycomb formed article can have the same expansion coefficient upon
firing, thereby inhibiting crack generation and improving durability.

[0047] Incidentally, the plugging portions may be formed after drying or
after drying and firing the honeycomb formed article by the method
described below.

[0048] 1-1-4. Firing:

[0049] The honeycomb-shaped substrate can be manufactured by finally
subjecting the honeycomb formed article having the plugging portions
formed therein to drying, calcining, and further firing.

[0050] There is no particular limitation on the drying method, and a
conventionally known drying method can be employed. For example, hot air
drying, microwave drying, dielectric drying, reduced pressure drying,
vacuum drying, and freeze drying may be employed. Of these, in that the
entire formed article can be dried quickly and uniformly, a drying method
where hot air drying and microwave drying or dielectric drying are
combined is preferable.

[0051] The calcination is performed in order to degreasing the organic
substances such as an organic binder, a pore former, and a dispersant
contained in the forming raw material (kneaded clay).

[0052] There is no particular limitation on the calcination conditions.
For example, the conditions of 550° C. for 3 hours in an ambient
atmosphere may be employed. Incidentally, the calcination conditions may
suitably be selected in accordance with the organic substances in the
forming raw material (kneaded clay). Generally, combustion temperature of
organic binders is about 100 to 300° C., and combustion
temperature of pore formers is about 200 to 1000° C. Therefore,
the calcination temperature can be 200 to 1000° C. In addition,
the calcination time is generally about 3 to 100 hours.

[0053] The firing is performed in order to secure predetermined strength
by sintering the framework particles and the like contained in the
forming raw material (kneaded clay) for densification.

[0054] Since the firing conditions differ depending on the forming raw
material (framework particles and the like in the forming raw material)
and the like, the conditions may suitably be selected in accordance with
the kind and the like of the forming raw material. For example, in the
case of firing a SiC powder and a metal Si powder in an Ar inert
atmosphere, the firing temperature is generally 1400 to 1500° C.
In addition, for example, in the case of firing a cordierite forming raw
material or an aluminum titanate raw material, the firing temperature is
preferably 1410 to 1440° C., and the firing time is preferably
about 3 to 10 hours.

[0055] A honeycomb filter manufactured by the method for manufacturing a
honeycomb filter of the present embodiment may be a honeycomb filter with
a catalyst loaded on the partition walls. There is no particular
limitation on the method for loading a catalyst on the partition wall,
and a conventionally known method can be employed. For example, a
catalyst having a mass ratio of alumina:platinum:ceria based material of
7:1:2 with the ceria based material having a mass ratio of Ce:Zr:Pr:Y:Mn
of 60:20:10:5:5 can be loaded on the partition walls by dipping, suction,
or the like. After that, for example, it is dried at 120° C. for 2
hours and baked at 550° C. for 1 hour to manufacture a honeycomb
filter with a catalyst.

[0056] 1-2. Step 1 (Deposition of Plugging Material Particles):

[0057] The step 1 is a step where the first particles (plugging material
particles) burning away due to a thermal treatment are deposited in the
open pores which are open on at least the inflow cell side of the surface
layer portion on the inflow cell side of the partition walls of a
honeycomb-shaped substrate to plug the openings of the open pores.

[0058]FIG. 4A is a cross-sectional view schematically showing a state of
depositing plugging material particles in the open pores which are open
on at least the inflow cell side of the surface layer portion on the
inflow cell side of the partition walls of the honeycomb-shaped substrate
in the step 1. In addition, FIG. 4B is an enlarged view of the P1
portion in FIG. 4A. Hereinbelow, the step 1 will be described with
referring to these figures.

[0059] In the present specification, the "surface layer portion on the
inflow cell side of the partition walls" is a layered portion (portion
shown by 12a) present on the inflow cell 11a side of the partition wall
12 as shown in FIG. 4B and having a thickness of 20% of that of the
partition walls 12. Incidentally, it is known that, as in the step 1 of
the present embodiment, when a solid-gas two-phase flow containing
microparticles passes through a particle-shaped layer filter like the
partition walls 12 of the honeycomb-shaped substrate, microparticles
derail from the air current due to dispersion or a particle-trapping
mechanism of interruption of microparticles to deposit in the surface
layer portion 12a of the particle-shaped layer filter (Regarding the
details of "particle-trapping mechanism of interruption", see "Y. Otani,
et. al, "Aerosol Science and Technology 10:463-474 (1989)").

[0060] 1-2-1. Plugging Material Particle:

[0061] There is no particular limitation on the plugging material
particles 21 as long as the particles burn away due to a thermal
treatment. Specific examples of the plugging material particles include
carbon black particles, graphite powder particles, acryl microparticles,
starch particles, polyethylene particles, polypropylene particles, nylon
particles, coke particles, cellulose particles, powder sugar, and phenol
particles. Of these, in view of easy conditions for the thermal treatment
for burning away the plugging material in the following step 3 (thermal
treatment), easy treatment of the gas generating due to the thermal
treatment, easy procurement, high cost-efficiency, and the like, carbon
black particles, graphite powder particles, acryl microparticles, starch
particles, polyethylene particles, polypropylene particles, nylon
particles, and coke particles are preferable, and carbon black particles,
graphite powder particles, acryl microparticles, and starch particles are
particularly preferable. Incidentally, here, "easy conditions for the
thermal treatment" means conditions where thermal treatment temperature
is not high and where no particular equipment, apparatus, and the like is
required because, for example, the atmosphere for the thermal treatment
is an ambient atmosphere.

[0062] There is no particular limitation on the number average particle
diameter of the plugging material particles 21 as long as the diameter is
not larger than the average pore size of the pores formed in the
partition walls 12 of the honeycomb-shaped substrate, and the
aforementioned "particles burning away due to a thermal treatment" can be
used. Since the number average particle diameter of the plugging material
particles 21 is not larger than the average pore size of the pores, many
plugging material particles 21 can enter the open pores 16 of the
partition walls 12. However, practically, the lower limit of the number
average particle diameter of the plugging material particles 21 is 0.01
μm (when the average pore size of the pores is 14 μm, it is 0.0007
times the average pore size), which is the minimum number average
particle diameter of the "particles burning away due to a thermal
treatment" which can be manufactured at present.

[0063] In addition, the number average particle diameter of the plugging
material particles 21 is particularly preferably not larger than 0.50
times the average pore diameter of pores. By setting the number average
particle diameter of the plugging material particles 21 to be not larger
than 0.50 times the average pore diameter of the pores, the plugging
material particles 21 easily enter the open pores 16 of the partition
walls 12 and easily deposit on the internal surface of the pores of the
partition walls 12. On the other hand, when the number average particle
diameter of the plugging material particles 21 is too small with respect
to the average pore size of the pores, it is predicted that the plugging
material particles 21 pass through the pores of the partition walls 12
and are discharged to the outflow cell 11b side to make deposition in the
pores of the partition walls 12 difficult. However, it has been confirmed
that even the plugging material particles 21 having a number average
particle diameter of 0.0007 times the average pore size of the pores has
an effect in plugging the open pores 16 of the partition walls 12.

[0064] 1-2-2. Deposition:

[0065] There is no particular limitation on the method for depositing the
plugging material particles 21, and a conventionally known method can be
employed. Above all, preferable is a method where a solid-gas two-phase
flow containing the plugging material particles 21 is splayed with an
ejector or the like to allow the particles to flow into the inflow cells
11a of the honeycomb-shaped substrate for deposition. At this time, it is
also preferable that air discharged to the outflow end 15b side is sucked
to introduce the plugging material particles 21 into the open pores 16 of
the partition walls 12 for deposition.

[0066] Since the pressure distribution is caused by the air current when a
solid-gas two-phase flow is allowed to flow into the inflow cells 11a,
the plugging material particles 21 themselves have an inertia force, and
therefore the plugging material particles 21 deposit nonuniformly, i.e.,
thinly in the inflow end portion 15a and thickly in the outflow end
portion 15b. However, by optimally setting the average pore size of the
pores of the partition walls 12, number average particle diameter of the
plugging material particles 21, spray amount, spray conditions, suction
conditions, and the like, the plugging material particles 21 are
deposited in at least the open pores 16 to plug the openings 16a of the
open pores 16, and thereby the surfaces of the partition walls 12 can be
made almost flat. This enables to deposit the membrane-forming particles
20 thinly and uniformly in the step 2 described below.

[0067] In addition, when the plugging material particles 21 are mixed with
air, that is, when a solid-gas two-phase flow is obtained, the plugging
material particles 21 in a fluidized state can be used. By using the
plugging material particles 21 in a fluidized state, aggregation of the
plugging material particles 21 can be inhibited, and clogging of the
pores can be inhibited. In addition, it is also possible to deposit the
plugging material particles 21 by generating a fluidized layer of the
plugging material particles 21 upstream of the honeycomb-shaped substrate
without spraying the plugging material particles 21 in a fluidized state
and sucking from the downstream.

[0068] 1-3. Step 2 (Deposition of Membrane-Forming Particles):

[0069] The step 2 is for depositing membrane-forming particles on the
surfaces of the partition walls where the openings of the open pores are
plugged in the step 1.

[0070]FIG. 5A is a cross-sectional view schematically showing a state of
further depositing the membrane-forming particles on the partition walls
where the openings of the open pores are plugged in the step 1. In
addition, FIG. 5B is an enlarged view of the P2 portion of FIG. 5A.
Hereinbelow, the step 2 is described with referring to these figures.

[0071] There is no particular limitation on the method for depositing the
membrane-forming particles 20 as long as the method can deposit the
membrane-forming particles 20 thinly and uniformly, and the same method
as the method for depositing the plugging material particles 21 described
above can be employed. Incidentally, the method for depositing the
membrane-forming particles 20 may be the same as or different from the
method for depositing the plugging material particles 21.

[0072] 1-3-1. Membrane-Forming Particle:

[0073] As the material for the membrane-forming particles 20, ceramic is
preferable. Specific examples of the material for the membrane-forming
particles 20 include oxide based ceramics such as cordierite, aluminum
titanate, mullite, alumina, zirconia, titania, spinel, zirconium
phosphate, aluminum titanate, and Ge-cordierite and nonoxide based
ceramics such as SiC and Si3N4.

[0074] Further, the material for the membrane-forming particles 20 is
preferably the same as that for the honeycomb-shaped substrate. That is,
it is preferable to use a pulverized product of a honeycomb-shaped
substrate, a further pulverized product of a processed powder generated
upon the cutting work in the process of manufacturing a honeycomb-shaped
substrate, or the like as the membrane-forming particles 20. By using
such membrane-forming particles 20, the honeycomb-shaped substrate and
the trapping layer have the same thermal expansion coefficient to be able
to inhibit exfoliation of the trapping layers from the partition walls
12. In addition, as the membrane-forming particle 20, framework particles
which are the raw material for the honeycomb-shaped substrate may be used
with no change.

[0075] Incidentally, there is no particular limitation on the method for
the pulverization, and a conventionally known pulverizing met hod can be
employed. However, wet pulverization is preferable. By the wet
pulverization, the membrane-forming particles 20 having a uniform
particle diameter can be formed, and aggregation of the membrane-forming
particles 20 can be inhibited.

[0076] Though there is no particular limitation on the number average
particle diameter of the membrane-forming particles 20, it is generally
0.1 to 50 μm, preferably 0.5 to 20 μm, and further preferably 1 to
15 p.m. When the number average particle diameter of the membrane-forming
particles 20 is below 0.1 μm, since the average pore size when the
trapping layer is formed is too small, pressure loss tends to be too
large because the air current passages becomes narrow. On the other hand,
when the number average particle diameter of the membrane-forming
particles 20 is above 50 μm, since the average pore size when the
trapping layer is formed is too large, PM easily passes, and it may be
impossible to obtain the original function of the trapping layer.

[0077] 1-4. Step 3 (Thermal Treatment):

[0078] The step 3 is a step of burning away the plugging material
particles by subjecting the honeycomb-shaped substrate where the plugging
material particles and the membrane-forming particles are deposited on
the partition walls.

[0079] FIG. 6A is a cross-sectional view schematically showing a state
after the plugging material particles are burnt away by subjecting the
honeycomb-shaped substrate where the plugging material particles and the
membrane-forming particles are deposited on the partition walls to a
thermal treatment in the step 3. In addition, FIG. 6B is an enlarged view
of the P3 portion of FIG. 6A. Hereinbelow, the step 3 will be
described with referring to these figures.

[0080] Since the plugging material particles 21 burn away due to a thermal
treatment, a thin and uniform layer of the membrane-forming particles 20
is formed on the surfaces of the partition walls 12. Generally, by the
subsequent firing, sintering is caused among the membrane-forming
particles 20 and between the membrane-forming particles 20 and the
partition walls 12 to be able to form a thin and uniform trapping layer
40.

[0081] Though there is no particular limitation on the conditions for the
thermal treatment as long as the plugging material particles 21 burns
away under the conditions, they are generally 200 to 1000° C. for
about 0.5 to 100 hours in an ambient atmosphere. Incidentally, the
temperature for the thermal treatment may suitably be selected depending
on the kind of the plugging material particles 21. For example, in the
case of carbon black particles or graphite powder particles, the
temperature may be about 800° C., and, in the case of acryl
microparticles or starch particles, the temperature may be about
500° C.

EXAMPLES

[0082] Hereinbelow, the present invention will be described specifically
on the basis of Examples. However, the present invention should not be
limited to these Examples. In addition, measurement methods for various
property values and evaluation methods for various properties are shown
below.

[0083] [Average Pore Size (μm) of Pores]

[0084] The average pore size of the pores formed in the partition walls
were measured by the mercury porosimetry using a mercury porosimeter
produced by Shimadzu Corporation.

[0085] [Average Membrane Thickness (μm) of Trapping Layer]

[0086] The "average membrane thickness (μm) of the trapping layer" was
obtained by the following measurement method.

[0087] In the first place, a honeycomb filter was cut to have a length of
10%, 50%, and 90% of the entire length from one end face. In each of the
three cross sections, three SEM images (nine in total) were taken under
the conditions of 500 magnifications using a scanning electron microscope
(trade name of "S-3200", produced by Hitachi, Ltd.). In each of the three
SEM image taken in one cross section, the membrane thickness of the
trapping layer was measured, and the average value of the membrane
thicknesses (measured values) of the three trapping layers measured was
obtained as the "membrane thickness of the trapping layer of the cross
section". In the same manner, regarding the other cross sections, the
"membrane thicknesses of the trapping layers of the cross sections" was
obtained. The average value of the three "membrane thicknesses of the
trapping layers of the cross sections" obtained above was defined as the
"average membrane thickness (μm) of the trapping layers".

[0089] Among the three "membrane thicknesses of the trapping layers of the
cross sections" obtained by the aforementioned method for measuring the
"average membrane thickness (μm) of the trapping layer", the
difference between the maximum "membrane thickness of the trapping layer
of the cross section" and the minimum "membrane thickness of the trapping
layer of the cross section" was defined as the "maximumminimum membrane
thickness difference (μm) of the trapping layer".

[0090] [Pressure Loss Reduction Rate (%)]

[0091] PM was deposited in the (clean) honeycomb filter where no PM was
deposited in such a manner that the PM amount per 1 L of the honeycomb
filter was 4 g. Into the honeycomb filter, air at 200° C. was
allowed to flow at a flow rate of 2.4 Nm3/min. The difference in
pressure between the upstream and the downstream of the honeycomb filter
was measured by the use of a differential pressure gauge, and the
measured value was defined as the pressure loss A. Using the measured
pressure loss, the "pressure loss reduction rate (%)" was obtained by the
following formula (1).

[0092] In the above formula (1), the pressure loss A0 is pressure
loss obtained by the use of a honeycomb filter with no trapping layer
formed therein (honeycomb filter of Comparative Example 1).

[0093] [Judgment]

[0094] The case satisfying all the following conditions 1 to 3 was judged
as "good", the case satisfying two conditions was judged as "fair", and
the case satisfying one or no condition was judged as "bad".

[0095] Condition 1: The average membrane thickness of the trapping layer
is 70 μm or less.

[0098] To 100 mass parts of the framework particles where a SiC powder and
a metal Si powder were mixed together at a mass ratio of 80:20 were added
6 mass parts of hydroxypropoxyl methyl cellulose as the organic binder,
mass parts of starch particles having an average particle diameter of 25
μm, and 35 mass parts of water as the dispersant, and they were
kneaded to prepare kneaded clay. Next, the kneaded clay was subjected to
extrusion forming using a die having a predetermined slit width where
cells having an octagonal cross-sectional shape (inflow cells) and cells
having a quadrangular cross-sectional shape (outflow cells) are
alternately formed to obtain a honeycomb formed article having a desired
size.

[0099] Next, the honeycomb formed article was dried with a microwave drier
and then completely dried with a hot air drier. Then, a mask was applied
on the inflow side end face of the outflow cells of the honeycomb formed
article, and the end portion on the side where the mask was applied
(inflow end portion) was immersed in the plugging material slurry
containing the aforementioned forming raw material for the honeycomb
formed article to form plugging portions in the inflow end portions of
the outflow cells. In the same manner, the plugging portions were formed
in the outflow end portions of the inflow cells.

[0100] A honeycomb formed article where plugging portions are alternately
formed in both the end portions was dried with a hot air drier and then
calcined at 550° C. for about 3 hours in an oxidation atmosphere.
Next, it was fired at 1450° C. for 2 hours in an Ar inert
atmosphere. Thus, there was manufactured a quadrangular prism shaped
honeycomb-shaped substrate having a length of 152 mm and a end face side
length of 36 mm with a cell density of 46.5 cells/cm2, a partition
wall thickness of 0.25 mm, an octagonal cross-sectional shape of the
inflow cells (the distance (length) between facing partition walls was
1.41 mm), a quadrangular cross-sectional shape of the outflow cells (the
distance (length of one side) between facing partition walls was 1.01
mm), an average pore diameter of 14 μm, and a porosity of 41%.

[0101] From the inflow end portion of the honeycomb-shaped substrate
manufactured above, carbon black particles having a number average
particle diameter of 0.01 μm as the plugging material particles were
injected so that the deposition amount per 1 L of the honeycomb-shaped
substrate might become 1 g.

[0102] Next, membrane-forming particles were injected in the same manner
as the injection of the plugging material particles. Incidentally, as the
membrane-forming particles, a SiC powder having a number average particle
diameter of 3 μm was used. Then, the plugging material particles were
burnt away by a thermal treatment at 800° C. for 2 hours in an
ambient atmosphere to manufacture a honeycomb filter of Example 1.

[0103] As results of various evaluations on the honeycomb filter of
Example 1, the average membrane thickness of the trapping layer was 40
μm, the maximumminimum membrane thickness difference was 30 μm, the
pressure loss reduction rate was 34%, and therefore the judgment was
"good". There results are shown in Table 1.

Comparative Examples 1 and 2

[0104] Each of the honeycomb filters was manufactured in the same manner
as in Example 1 except that neither a plugging material particle nor a
membrane-forming particle was disposed in Comparative Example 1 and that
no plugging material particle was disposed in Comparative Example 2. The
results of various evaluations on each of the filters are shown in Table
1.

Examples 2 to 10, Comparative Examples 3 and 4

[0105] Each of the honeycomb filters was manufactured in the same manner
as in Example 1 except that the kinds and the number average particle
diameters of the plugging material particles shown in Table 1 were
employed. That is, each of the honeycomb filters were manufactured by the
use of carbon black particles having a number average particle diameter
of 0.05 μm in Example 2, carbon black particles having a number
average particle diameter of 0.10 μm in Example 3, carbon black
particles having a number average particle diameter of 0.14 μm in
Example 4, acryl microparticles having a number average particle diameter
of 0.60 μm in Example 5, acryl microparticles having a number average
particle diameter of 1.4 μm in Example 6, acryl microparticles having
a number average particle diameter of 2.6 μm in Example 7, starch
particles having a number average particle diameter of 3.0 μm in
Example 8, graphite powder particles having a number average particle
diameter of 7.0 μm in Example 9, graphite powder particles having a
number average particle diameter of 10 μm in Example 10, graphite
powder particles having a number average particle diameter of 20 μm in
Comparative Example 3, and starch particles having a number average
particle diameter of 30 μm in Comparative Example 4. The results of
various evaluations on each of the honeycomb filters are shown in Table
1.

[0106] Each of the honeycomb filters was manufactured in the same manner
as in Example 3 except that cordierite forming raw material was used in
Example 11 and that aluminum titanate raw material was used in Example 12
as the raw material for the honeycomb-shaped substrate and the
membrane-forming particles. The results of various evaluations on each of
the honeycomb filters are shown in Table 2.

[0107] From Tables 1 and 2, it is clear that a honeycomb filter obtained
by forming a trapping layer on the surfaces of the partition walls which
are made almost flat by the use of plugging material particles having a
number average particle diameter not larger than the average pore size of
the pores formed in the partition walls and then burning away the
plugging material particles is provided with a thin and uniform trapping
layer. Further, it is clear that, in the case of using plugging material
particles having a number average particle of at most 0.50 times the
average pore size of the pores formed in the partition walls, the
honeycomb filter is provided with a thin and uniform trapping layer and
has high pressure loss reduction rate.

[0108] A honeycomb filter of the present invention can suitably be used as
a filter for trapping and removing particulate matter contained in
exhaust gas discharged from internal combustion engines such as a vehicle
engine, a construction machine engine, and an industrial machine
stationary engine, other burning appliances, and the like.

Patent applications by NGK Insulators, Ltd.

Patent applications in class Against inner surface of a hollow preform or solidified layer

Patent applications in all subclasses Against inner surface of a hollow preform or solidified layer